Terrestrial mobile telecommunications systems are well known and a number of different systems have developed which operate according to different standards. These public land mobile networks (PLMNs) may operate according to analogue or digital standards. In Europe, the Far East, excluding Japan and elsewhere, the Global System Mobile (GSM) network has become popular, whereas in USA, the Advanced Mobile Phone Service (AMPS) and the Digital American Mobile Phone System (DAMPS) are in use, and in Japan, the Personal Handiphone System (PHS) and the Personal Digital Communication (PDC) network are used. More recently, proposals have been made for a Universal Mobile Telecommunications System (UMTS). These networks are all cellular and land-based with transmitter/receivers which communicate with mobile user terminals. Considering for example the GSM system, individual cells of the mobile network are served by a series of geographically spaced, terrestrial base transceiver stations (BTSs) which are coupled through base station controllers (BSCs) to a mobile switching center (MSC) which may provide a gateway out of the network to a conventional public switched telephone network (PSTN). The PLMN includes a home location register (HLR) which stores information about the subscribers to the system and their user terminals. When a user terminal is switched on, it registers with the HLR. If the user roams to a different GSM network, the user terminal registers with a visitor location register (VLR) of the visited network, which communicates with the HLR of the home network for routing and other purposes. DAMPS, PHS and PDC networks include similar location registers. More recently, a number of different mobile telecommunication systems have been proposed that use satellite communication links to the mobile user terminals.
One network known as the IRIDIUM.TM. satellite cellular system is described in for example EP-A-0365885 and U.S. Pat. No. 5,394,561 (Motorola), which makes use of a constellation of so-called low earth orbit (LEO) satellites, that have an orbital radius of 780 km. Mobile user terminals such as telephone handsets are configured to establish a link to an overhead orbiting satellite, from which a call can be directed to another satellite in the constellation and then typically to a ground station which is connected to conventional land-based networks.
Alternative schemes which make use of so-called medium earth orbit (MEO) satellite constellations have been proposed with an orbital radius in the range of 10-20,000 km and reference is directed to Walker J. G. "Satellite Patterns for Continuous Multiple Whole Earth Coverage" Royal Aircraft Establishment, pp 119-122 (1977). These orbits are also known as intermediate earth orbits (IEOs). As examples, reference is directed to the ICO.TM. satellite cellular system described for example in GB-A-2 295 296, and to the ODYSSEY.TM. satellite cellular system described in EP-A-0 510 789. With both the ICO.TM. and the ODYSSEY.TM. systems, the satellite communication link does not permit communication between adjacent satellites and instead, a signal from a mobile user terminal such as a mobile handset is directed firstly to the satellite and then directed to a ground station or satellite access node (SAN), connected to conventional land-based telephone network. This has the advantage that many components of the system are compatible with known digital terrestrial cellular technology such as GSM.
In satellite communications networks, ground stations are located at different sites around the world in order to communicate with the orbiting satellites. In the ICO.TM. system and others, a visitor location register is associated with each of the satellite ground stations, which maintains a record of the individual user terminals that are making use of the particular ground station.
In certain areas of the world, coverage provided by a conventional terrestrial PLMN and the satellite network will overlap. It has been proposed that the individual mobile terminals be selectively operable with either a terrestrial PLMN or the satellite network. The user terminals may include a switch to allow the user to select the network or alternatively, an automatic selection may be made e.g. on the basis of signal strength or other factors. Thus, the user of the terminal can use the satellite network as a roamed network, with billing and other usage information being passed back to the Home PLMN for billing to the customer. It is envisaged that normally, the conventional terrestrial PLMNs will be preferred when available, for reasons of cost and signal strength. Nevertheless in some circumstances, the satellite network may provide services which are not available through the Home PLMN and so it would be desirable to establish service interworking between the networks so that roaming users from terrestrial PLMNs may make use of unique services provided by the satellite network, when they roam onto the satellite network.